Cells exhibit a wide variety of physiological functions in response to various exogenous stimuli such as neurotransmitters, hormones and growth factors. In this regard, calcium ions play an important role as a messenger for intracellular signaling. Main calcium sources are intracellular calcium stores and extracellular fluids. Calcium is released from an intracellular calcium store through an inositol 1,4,5-triphosphate (IP3) receptor that is a second messenger or through a ryanodine receptor that is insensitive to IP3 but releases calcium in accordance with an increase in the intracellular calcium concentration.
IP3 as an intracellular second messenger carries out IP3-induced calcium (Ca) release (IICR) to induce liberation of calcium ions from an intracellular calcium ion pool. An IP3 receptor is an intracellular calcium ion release channel activated by binding to IP3. The IP3 receptor forms a gene family, exhibits various functions, tissue- or cell-specific expressions and intracellular localizations, and plays an essentially important role in the biological functions.
It is known that IP3 is produced in a pathway of activating various receptors coupled with G-proteins or in a pathway of activating various receptors coupled with tyrosine kinase activity. Phospholipase C activated in the above pathway decomposes phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers, IP3 and diacylglycerol (DG). IP3 binds to an IP3 receptor present in an intracellular calcium store to release calcium. On the other hand, DG together with the calcium activates protein kinase C to control various physiological functions.
It is known that various channels are involved in calcium ion entry from an extracellular fluid. The channels are roughly classified into voltage-dependent channels activated in accordance with the potential of a cell membrane and channels activated irrespective of the potential. Calcium permeable neurotransmitter receptors (such as NMDA receptors) are known as the latter channels. Recently, calcium permeable channels activated by activation of G-protein coupled receptors or tyrosine kinase receptors and receptor activated calcium channels (RACC) have been attracted attention. RACCs include capacitative calcium entry (CCE) channels, second messenger response channels and G-protein response channels.
A CCE channel is activated when calcium ions are released from and depleted in an intracellular calcium store, and has a function of allowing entry of extracellular calcium ions to refill the intracellular calcium store with calcium ions. For this reason, the CCE channel is also called store operated calcium entry channel (SOC).
The presence of this channel has been electrophysiologically revealed mainly in nonexcitable cells such as immunocytes, vascular endothelial cells and platelets, and the channel is known as a main calcium entry pathway in nonexcitable cells. However, the molecular entity of the channel has not been clear. Further, the mechanism of recognition of depletion in the intracellular calcium store and the mechanism of activation have not been clear.
However, it has been verified in the experiments shown below and the like that capacitative calcium entry and calcium release from the intracellular calcium store caused by involvement of the aforementioned IP3 play an important role in expression of functions of cells.
(1) When platelets are stimulated by thromboxane A2, thrombin or the like, the platelets are aggregated through IP3 and thrombi are formed, resulting in ischemic heart or brain disease. In this regard, it is known that capacitative calcium entry subsequent to IP3-induced calcium release (IICR) is also essential to platelet aggregation Biochimica et Biophysica Acta, 1082, 219-238 (1991); Platelets, 11(4), 215-21 (2000)].
(2) Helper T cells (Th1) of subset 1 in T-lymphocytes produce and secrete cytokines such as interleukin 2 (IL-2) and interferon γ in accordance with activation by antigen presenting cells to express IL-2 receptors. In this regard, NF-AT as an enhancer must become active and be transferred into nuclei in order to start transcription of IL-2 genes. It is known that an increase in the intracellular calcium concentration by capacitative calcium entry is essential for an activation of the NF-AT [J. Cell Biol., 131(3), 655-67 (1995)].
(3) IP3 is produced and calcium is released by stimuli from leukotriene D4 (LTD4), angiotensin II or the like, so that bronchial smooth muscle and vascular smooth muscle contract to cause asthma, hypertension, cerebral vasospasm or the like. In this regard, it is known that capacitative calcium entry is also essential [J. Pharm. Exp. Ther., 244, 508-515 (1987); Protein Nucleic Acid and Enzyme, 36, 885-895 (1991); J. Membr. Biol., 155(1), 61-73 (1997)].
(4) In exocrine pancreatic cells, the intracellular calcium concentration is increased through IP3 by stimuli from cholecystokinin, acetylcholine or the like and abnormal secretion of protease occurs to cause pancreatitis. In this regard, it is known that capacitative calcium entry is also essential [Pharmacology & Toxicology, 68, 83-87 (1991); Proc. Natl. Acad. Sci. USA, 97(24), 13126-13131 (2000)].
(5) Leukotriene B4 (LTB4) produced from neutrophils increases the intracellular calcium concentration through IP3 and causes migration of the neutrophils to the inflammatory site to develop inflammation [ANN. NY. ACAD. Sci., 524, 187-195 (1988)]. LTB4 production is also involved in expansion of the necrotic layer in myocardial infarction [J. Pharm. Exp. Ther., 228, 510-522 (1983)].
(6) In the kidneys, stimuli from angiotensin II, bradykinin or the like produce IP3 and proliferate mesangial cells to cause glomerulonephritis. IP3 also has an influence on various other renal diseases [Metabolism, 27, 413-425 (1990)].
In recent years, it has been clear that capacitative calcium entry has an important function not only in the nonexcitable cells described above but also in neurons. For example, it is known that presenilin known as a gene responsible for familial Alzheimer's disease has a function as γ-secretase cleaving amyloid precursor protein. It has been clear that capacitative calcium entry is abnormal in culture cells when expressing presenilin discovered in a familial Alzheimer's disease patient into which a point mutation is introduced [Neuron, 27(3), 561-72 (2000)]. It has also been clear that capacitative calcium entry is abnormal in an experiment using mouse-derived primary culture cells having presenilin genes destroyed [J. Cell Biol., 149(4), 793-8 (2000)].
As described above, endogenous calcium and capacitative calcium entry are extremely highly associated with various diseases.
Accordingly, it is assumed that endogenous calcium release inhibitors or capacitative calcium entry inhibitors have an action of inhibiting an increase in the intracellular calcium concentration and are therefore useful as prophylactic and/or therapeutic agents for diseases such as platelet aggregation, ischemic heart or brain disease, immunodeficiency, allergic disease, bronchial asthma, hypertension, cerebral vasospasm, various renal diseases, pancreatitis or Alzheimer's disease.
JP Patent No. 2987727 discloses (2-aminoethoxy)diphenylborane and tetraphenyldiboroxane (tetraphenyldiboroxide) having an effect of inhibiting calcium release from an endogenous calcium store by mechanisms of IICR and calcium induced calcium release (CICR).
It is also described that (2-aminoethoxy)diphenylborane has an SOC inhibitory action through an IP3 receptor inhibitory action [Science, 287, 1647-1651 (2000)].
Further, WO 03/033002 describes that bis-1-oxaquinolizidine, xestospongin C, xestospongin A, araguspongin B and the like are useful as inhibitors for calcium channels through IP3 receptors.
In such a situation, it can be greatly expected that a drug reducing the intracellular calcium concentration abnormally increased by IP3 receptor activation or capacitative calcium entry is useful for prevention or treatment of the various above-described diseases caused by an increase in the intracellular calcium concentration, if such a drug can be developed.
To attain such an object, the present inventors have found a certain 2-APB derivative as an intracellular calcium concentration control agent having activity stronger than that of (2-aminoethoxy)diphenylborane (2-APB) and filed an international application (WO 03/033002).